1. Field of the Invention
The present invention relates generally to methods of synthesizing TFPX, and more specifically to such methods which utilize S.sub.N 2 nucleophilic substitution reactions.
2. Discussion of the Related Art
Dielectric films are widely used throughout both the electronics and coatings industries. Due to their relatively high dielectric constants and melting points, there is an increasing interest in forming dielectric layers from parylene polymers having the molecular structure: ##STR1##
wherein X is typically a hydrogen atom or a fluorine atom.
Parylene polymers are usually formed by chemical vapor deposition processes. One such process is the Gorham process in which a parylene dimer having the molecular structure: ##STR2##
is vaporized and the dimer bonds are then cleaved to yield parylene monomers. The parylene monomers are deposited onto a surface and subsequently polymerized. Because the dielectric constant and melting temperature of parylene polymers usually increases as the number of fluorine atoms within the polymer increases, it is desirable to use octafluoro-[2,2]paracyclophane (hereinafter "AF4") having the molecular structure: ##STR3##
as the parylene dimer. .alpha.,.alpha.,.alpha.',.alpha.'-tetrafluoro-p-xylene (hereinafter "TFPX") having the molecular structure: ##STR4##
is often used as the starting material in AF4 synthesis. The conventional procedure for synthesizing TFPX involves the fluorination of terephthaldehyde, which has the molecular structure: ##STR5##
SF.sub.4 and MoF.sub.6 are the most commonly used reagents for terephthaldehyde fluorination. However, SF.sub.4 and MoF.sub.6 are expensive, reducing the industrial utility of this synthetic scheme. In addition, SF.sub.4 and MoF.sub.6 are toxic materials, so a large amount of hazardous waste is produced using these reagents.
Russian Patent No. 2,032,654 discloses an alternate method of synthesizing TFPX in which .alpha.,.alpha.,.alpha.',.alpha.'-tetrabromo-p-xylene (hereinafter "TBPX") having the molecular structure: ##STR6##
is reacted with SbF.sub.3 to produce TFPX. This method employs the well established electrophilic catalyzed S.sub.N 1 reaction mechanism for replacement of benzylic halogen atoms of the TBPX with fluorine atoms. According to this method, the antimony atom in SbF.sub.3 acts as an electrophile which removes bromine from TBPX to form a carbocation. The carbocation subsequently reacts with fluorine to form TFPX. While this reaction is reported to provide a good yield when carried out under comparatively mild reaction conditions, antimony containing compounds are highly toxic and expensive. Furthermore, the SbF.sub.3 is used in a stoichiometric amount rather than a catalytic amount, resulting in large quantities of hazardous waste materials. Therefore, this method of synthesizing TFPX has limited use for industrial applications.
Therefore, it remains a challenge in the art to synthesize TFPX using a method that is comparatively inexpensive and results in a reduced amount of hazardous waste.